<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>2 bfc</title>
  <style>
    aside {
      width: 100px;
      height: 100px;
      background: gold;
      float: left;
    }

    main {
      height: 200px;
      background: orange;
      overflow: hidden;
      margin-left: 120px;
    }
  </style>
</head>

<body>
  <aside></aside>
  <main></main>
</body>

</html>